1. Field of the Invention
The present invention relates to a method of manufacturing a laminate such as a multilayer printed wiring board or the like, and the improved structure of a grommet used for this method.
2. Description of the Related Art
FIG. 5 is a perspective view showing a conventional grommet, and FIG. 6 is a cross-sectional view showing a conventional multilayer printed wiring board using the grommet shown in FIG. 5.
As disclosed in Japanese Patent Application Laid-open No. Hei-6-244555 titled as "METHOD OF MANUFACTURING MULTILAYER LAMINATE PLATE", the conventional multilayer printed wring board is obtained by laminating an inner layer wiring plate 1 having a wiring circuit formed on the surface thereof and a prepreg 2 to form a laminate 5 and then subjecting the laminate to a forming process under heat and pressure to settle the laminate.
In order to avoid positional displacement or missregistration during the heating and pressuring process, the inner wiring plate 1 and the prepreg 2 are temporarily fixed by a grommet 3 which is provided at the peripheral portion of the inner wiring plate 1 and the prepreg 2 so as to penetrate through the inner layer wiring plate 1 and the prepreg 2.
FIG. 5 is a perspective view showing the outlook of the grommet 3 used for the temporary fixing. As shown in FIG. 5, the grommet 3 has a flange 4 at the tip thereof and a number of cut-out grooves (not shown) in the longitudinal direction at the upper and lower end portions. A pressure is applied to the laminate 5 from the upper and lower sides by the flange 4 to fix the laminate 5, whereby the inner layer wiring plate 1 and the prepreg 2 are temporarily settled and the inner layer wiring layer 1 and the prepreg 2 are positioned.
Subsequently, resin such as epoxy resin or the like which is hardened at high speed is filled into a through hole 6 of the grommet by an injector or the like, and the resin thus injected is hardened to form a resin layer 7 in the through hole. Thereafter, metal foil 8 such as copper foil or the like is laminated on the laminate, and the laminate with the metal foil is set in a press forming apparatus (laminating press) while it is sandwiched between forming plates to be subjected to the heat and pressure forming process.
By performing the above heat-and-pressure forming process, the prepreg resin is completely hardened to obtain a multilayer printed wiring board having the inner layer wiring plate 1.
Japanese Patent Application Laid-open No. Hei-9-293965 titled as "MULTILAYER WIRING BOARD MANUFACTURING METHOD" describes a technique of filling solder in place of the resin layer to be formed in the through hole of the grommet pin disclosed in Japanese Laid-open Patent Application No. Hei-6-244555.
Further, Japanese Patent Application Laid-open No. Hei-2-10888 titled as "MULTILAYER WIRING BOARD MANUFACTURING METHOD" describes a technique of temporarily fixing a laminate comprising plural inner layer wiring plates and plural prepregs by a grommet pin which is formed so as to be hollow at a crimped or grooved portion and so as to be closed with metal or the like at portions other than the crimped or grooved portion.
However, when the conventional grommet is used and only resin in the prepregs which are overlaid on the grommet from the upper and lower sides is filled into the grommet, the resin cannot be completely filled into the grommet, and thus a work of filling the resin into the grommet hole in advance is needed. Further, a step of hardening the filled resin is required, so that the total number of steps is increased.
When the thickness of the laminate to be manufactured is equal to 1.6 mm or more, it is difficult to completely fill the resin into the grommet. Therefore, insufficient filling occurs and a filling-insufficient portion 11 occurs as shown in FIG. 6. Accordingly, when the heat-and-pressure forming process is carried out, the sufficient strength cannot be obtained at the grommet portion, and thus the respective inner layer members are positionally displaced or missregistered from one another by 150 .mu.m or more.
The reason for this is as follows. The conventional grommet is designed in a cylindrical shape, and when a prepreg to be mounted after the temporary fixing is thin, the resin cannot be completely filled into the grommet hole. Therefore, cavities occur in the grommet hole and thus the sufficient strength cannot be obtained in the heat-and-pressure forming process, so that the work of filling the resin into the grommet hole is needed.
Further, when the resin is injected into the grommet by an injector or the like as in the case of the conventional technique, the resin may be swollen in case that the excess amount of the resin is injected, so that the surface thereof is not flat. Accordingly, uniform pressure is not applied in the heat-and-pressure forming process and thus unevenness occurs in the final product. Therefore, in the prior art, grommet portions which are provided at four corners on the periphery of the laminate having a lamination blank size (the size at the lamination time) are cut off together with the peripheral portion after the heat-and-pressure forming process, thereby obtaining the design blank having a design blank size (product size) having no unevenness. Therefore, although any grommet does not remain in the products, it is needed to increase the lamination blank size to be larger than the design blank size by about 10%, so that material may be wasted or much labor is imposed on the cut-off work of the peripheral portion.